Ch. i. south AMERICA. 17 



retried in 1744, from other obfervatioiis and journals 

 byMeff. Montaine and Dodlbn of London, I ihall add 

 lome refle6lions, in order to expofe the negligence in 

 conftruiting the magnetic needles, i. It appears that 

 the variations obferved by Don George Juan do not 

 agree with mine, which is not to be attributed to a de- 

 fect: in the obfcrvations. This is fufficiently evident 

 from comparing them. The differences between thofe 

 obferved by Don George and thofe on the chart, arq 

 nearly evcvy where uniform ; the moll confidcrable be* 

 ing a degree and 30 minutes; one making the variation 

 a° 30', and the other a degree only. This probably 

 arofe from the motion of the fliip, which hinders the 

 needle from being entirely at reft; or from the diik ot 

 the fun, by reafon of intervening vapours, not being ac- 

 curately determined, or fome other unavoidable acci- 

 dent ; the error, when the diiference is lefs than a de- 

 gree, being fcarce perceivable in thefe obfervations. 

 Thus, on a medium, the rational concluiion is, that the 

 needle ufed in thefe obfervations varied a degree and 40 

 minutes lefs than thofe when the map was conilrucled. 



The fame uniformity appears in the differences be- 

 tween my obfervations and the chart; but «it muft be 

 oblerved, that having ufed two different needles, the 

 particulars of each nearly correfpond, fo that betv^ecn 

 the five firft, the greatelj difference is of 40 minutes, 

 which intervene between the fraalleil diiference of 

 2,° 50', and the grcatell of 3° 30'. Hence, taking the 

 medium between both, the diiference between my ob- 

 fervations and the chart will be 3° 16', the latter being 

 Í0 much lefs than the former. The three lail do not 

 want this operation, the difference of l° 30' being 

 equal in all, and the variations refulting from thefe ob- 

 fervations are alio lefs than thofe delineated on the 

 map ; the variation having paifed to a different fpe- 

 cies; namely, from N. W. toN. E. Thisdemonilrates, 

 that the firil needle I made ufe of, whether it had been 

 ill touched or the ñeel not accurately placed, varied 

 1° 30' wefterly lefs than that ufed by Don George 



Vol, L C Juan ; 



